Spectrometry is a proven technique for measuring the concentration of skin constituents like melanin and hemoglobin through the spectrum emitted or absorbed by them. It has been used to study the interaction between radiation and matter as a function of wavelength or frequency. A drawback of this technique is that the measurements are typically done on a very small tissue area using a point source-detector pair. For a larger field of view or area-of-interest, either multispectral imaging or time/frequency resolved measurements have been used. In typical implementations of these techniques, measurements are evaluated against an assumed light-tissue interaction model and the unknown optical properties of the underlying tissue are solved as an inversion problem. While the aforementioned techniques can provide true quantitative, and in some cases even depth-dependent (i.e., three-dimensional) information, the instrumentation and processing required are complex.
Conventional reflectance imaging of skin typically entails illuminating the skin with white light and capturing the light reflected therefrom. (A conventional reflectance image may also be referred to as a standard captured image, or a total reflection image among other commonly used terms.) The reflected light has two components: a specular or surface reflection component, and a diffuse reflection component. When separated from each other, each component provides useful information about the imaged tissue. The surface reflection component is useful for analyzing topological characteristics of tissue such as surface texture and visible features such as wrinkles and pores. The diffuse reflection component, which is due to light that has interacted with the tissue interior, conveys information about the optical properties of the tissue such as the distribution of chromophores like melanin and hemoglobin. Some photons of the incident light penetrate within the tissue and undergo multiple scattering and absorption events before some of those photons are backscattered as diffuse reflected light. The average penetration depth of a photon is dependent on its wavelength, with longer-wavelength photons penetrating deeper into the tissue. The wavelength- (or color-) dependent average penetration depth of photons is illustrated in FIG. 1.
Cross-polarized and parallel-polarized imaging have been the preferred techniques used for respectively capturing diffuse and surface reflection components independently. In a typical implementation illustrated in FIG. 1, a broad-band light source 10 illuminates a skin sample while images are captured using a digital color camera 20 with polarizing filters 15, 25 in both the illumination and reflection paths. For capturing diffuse reflection images, the polarization axes of filters 15, 25 are oriented perpendicular to each other. A part 51 of the polarized incident light 50 is reflected back from the skin surface. This surface reflection component 51 maintains the same polarization as that of the incident light 50 and is blocked by the detector side polarizing filter 25 due to its orthogonal orientation relative to filter 15 in the illumination path. Another part 52 of the polarized incident light 50 penetrates the skin surface and undergoes multiple scattering and absorption events before some of those photons are backscattered as diffuse reflected light 53. Due to scattering, the diffuse reflected light 53 has lost all its polarization and hence a portion thereof is able to pass through the detector side polarizing filter 25. For capturing surface reflection images, the polarization axes of the filters 15, 25 are oriented parallel to each other.
In addition to cross-polarized imaging as described above, diffuse reflection images can also be captured using a dark field illumination technique where the light is incident at an angle only on the edge of the skin area being imaged while the detector is allowed to only capture reflected light which is almost perpendicular to the skin surface. Although dark-field illumination techniques do not require polarizing filters, they have several drawbacks. The angle of illumination is dependent on the area being illuminated. If the angle of incidence is too shallow or too direct, then there will be a dark spot in the center where no light has reached. The area that can be imaged is very small since it will have a radius equal to the average of the total scattering length of the light in tissue. Some form of a ring light source of appropriate diameter is thus required.
Diffuse reflection images, obtained using either cross-polarized imaging or dark-field illumination techniques, have been analyzed for the purpose of evaluating tissue pigmentation and distribution information. Evaluation of diffuse reflection color images for tissue pigmentation information has been carried out using various tools such as: 1) color-space transformations; 2) various combinations of color-spaces; 3) optical models of light-tissue interaction, treating the three color channels of the images as multi-spectral measurements; and 4) principle component analysis (PCA) or independent component analysis (ICA) techniques with or without a linear or non-linear tissue absorption models.
Another technique for evaluating tissue pigmentation, developed by Canfield Imaging Systems, is the RBX technique which can transform Red/Green/Blue (RGB) cross-polarized skin images into Red and Brown representations indicative of hemoglobin and melanin distributions, respectively. (See R. Demirli et al., “RBX Technology Overview”, Canfield Imaging Systems, February 2007.) In an implementation of the RBX technique, a RGB cross-polarized image of skin is transformed to a Red/Brown/X (RBX) color-space using a combination of a light transport model of skin and a spectral-dependent model of the source-detector configuration used in capturing the RGB image. The RBX color-space transformation is based upon random samplings of cross-polarized facial skin images obtained from a large population of patients with different skin types.
Due to restrictions on imaging geometry, image acquisition and processing, quality of image acquisition and illuminating optical components, polarizing filter misalignment, and/or calibration errors, it may not always be possible to capture a pure diffuse reflection image using cross-polarized imaging. The quality of the cross-polarized data tends to be compromised when using shallow angles for illumination or near-perpendicular angles for detection with respect to the imaging surface, such as when capturing multiple images from various angles for 3D imaging using stereo techniques. When the field-of-view is large, cross-polarization is compromised as one moves away from the central axis of the imaging plane. Cross-polarization is also compromised if the area being imaged is not flat but has appreciable curvature. The resultant data in either case is similar to that of a standard captured image which has both specular and diffuse reflection information. This limits the ability to obtain tissue pigmentation information using techniques such as the RBX and other techniques described above.
Good quality cross-polarized images can be obtained with closed imaging systems such as Canfield's VISIA imaging systems which provide a well controlled illumination and image capture environment. Cross-polarized images captured with such a system are of sufficient quality for obtaining good tissue pigmentation information using techniques such as the RBX and other techniques described above. Without such systems, however, it typically is not possible to obtain diffuse reflection images of sufficient quality from which good tissue pigmentation information can be extracted.